Kenyans observed three days of national mourning after the attack. Photo: AFP

Paramedics help a student injured during the attack by al-Shabaab extremists. Photo: AFP

Kenyan army spokesman David Obonyo told the BBC that the military had responded to "threats" by launching the air strikes on Sunday night in the remote region.

Two camps had been destroyed, he said, adding: "The bombings are part of the continued process and engagement against al-Shabaab, which will go on."

But an eyewitness, speaking to BBC Somali, said the attack had wounded three civilians, and destroyed livestock and wells in an area without an al-Shabaab presence.

The attack on Garissa University, about 150km from the Somali border, was the deadliest by al-Shabaab in Kenya.

The al-Qaeda affiliate says it is at war with Kenya, and wants it to withdraw troops sent to Somalia in 2011 to help the weak government in Mogadishu fight the militants.

Calls for refugee camp closure

Governors and MPs from north-eastern Kenya have called for the closure of the Dadaab refugee camp, where about 500,000 people who fled conflict in Somalia are taking shelter.

They told a news conference in the capital, Nairobi, that the camp was used by al-Shabaab as a training and coordination centre.

Aid agencies have rejected previous calls for the closure of Dadaab, the largest refugee camp in Africa.

An MP in Garissa, Aden Duale, said Kenya should "engage" with the international community to step up patrols along its long and porous border with Somalia.

A Kenyan soldier standing guard at the Garissa University campus. Photo: AFP

Red Cross workers help a relative of one of the students killed by al-Shabaab gunmen. Photo: AFP

Meanwhile, Kenya's government has denied accusations that its security forces were slow to respond to last week's assault on the university.

Mr Kenyatta's spokesman Manoah Espisu told the BBC that the military was at the scene within minutes of the attack, and had helped save the lives of many students on campus.

Local media reported that it took special forces several hours to arrive at the university because of delays in their flight from Nairobi.

The attack ended when the four militants were killed by police more than 15 hours after they stormed the university.

One of the gunmen has been named as Abdirahim Abdullahi, a law student who graduated from Nairobi University in 2013.

His father is a local chief, and had reported his son missing, according to local media.

A former fellow student of Abdullahi told BBC Newsday that he had been a "charming fellow" who did not show any sign of holding militant views at university.

"He was very intelligent ... I was very shocked that a person I sat with in class - what would drive someone to change so much?" said the former student, who asked not to be identified for fear of reprisals.